1. Field of the Invention
The invention relates generally to GPS receivers and more particularly to a GPS receiver having a fast acquisition of a GPS signal having a low signal-to-noise ratio.
2. Description of the Prior Art
Global positioning system (GPS) receivers have been used for several years for determining geographical location and/or time in commercial applications including navigation, timing, mapping, surveying, machine and agricultural control, vehicle tracking, and marking locations and time of events. Given such wide commercial application, it is clear that GPS receivers provide a good value for many users. However, the global positioning system has been limited in several potential applications because existing GPS receivers are unable to acquire a GPS signal unless the GPS signal has signal-to-noise that is greater than a certain level. Typically, this is not a problem where the GPS receiver is mounted on a platform such as a ship, airplane, farm tractor, or a vehicle traveling on an open highway. However, the signal-to-noise limitations of GPS receivers make it generally impractical to use GPS indoors or where the GPS signal may be blocked by buildings or trees. For example, it might be desirable to obtain a GPS-based location with a handheld GPS receiver indoors or beneath foliage, or with a vehicle mounted GPS receiver in an urban canyon. Further, in order to obtain a high enough signal level, GPS receivers are used with specially designed hemispherical GPS antennas that are positioned to point upward with a clear view toward the sky. This can be inconvenient for the user of a handheld GPS receiver. For example an E911 requirement for cellular phones has mandated that it be possible to determine the location of the phone to within one-hundred twenty-five meters within about five seconds some high percentage of the time. These requirements might be met by a GPS receiver in the cellular phone where the phone was limited to use outside of buildings or automobiles by a user wearing a GPS antenna on his hat or shoulder pad. The GPS antenna would then somehow be connected from the hat or shoulder pad to the GPS receiver in the cellular phone. However, it is not expected that such an approach would be acceptable except in special circumstances. In order to meet the needs of these and other similar applications, there is a need for a GPS receiver having a fast signal acquisition for a signal having a low signal-to-noise.
It is therefore an object of the present invention to provide a global positioning system (GPS) receiver having a rapid acquisition of an incoming GPS signal having a low signal-to-noise ratio by combining epochs in chunks of GPS signal code data.
Briefly, in a preferred embodiment, a system of the present invention includes a base station and a GPS receiver. The GPS receiver includes a transceiver for radio communication with the base. The GPS receiver enters an operational mode from a low power standby mode when a user keys the GPS receiver to transmit a radio signal to the base or a radio signal from the base awakens the GPS receiver. The base transmits a radio signal having an accurate frequency and information for time, location, GPS Doppler frequency shifts, and GPS data to the GPS receiver. The GPS receiver uses the radio signal frequency and the GPS information for acquiring the GPS signal and computing GPS pseudoranges. The transceiver then transmits the pseudoranges in a radio signal back to the base and the GPS receiver returns to the standby mode. The base uses the pseudoranges for computing the geographical location of the GPS receiver.
The GPS receiver in a preferred embodiment includes a GPS signal sample memory, a digital signal processor, a microcontroller, and a reference frequency generator. The microcontroller and the transceiver determine a frequency adjustment based upon the information in the radio signal and a comparison of the radio signal frequency with a reference frequency generated by the reference frequency generator. The GPS signal sample memory receives and stores raw GPS signal samples for a sample time period of preferably about one second. The digital signal processor uses the frequency adjustment and subsequent frequency corrections determined within the GPS receiver for processing and re-processing the same stored raw GPS signal samples in order to provide successively more accurate correlation times. The microcontroller uses the correlation times for determining and providing successively finer frequency corrections as feedback to the digital signal processor. When the frequency adjustments and corrections have caused a local frequency to match the frequency of the incoming GPS signal to within a selected threshold, the microcontroller uses a correlation time for computing a GPS pseudorange. Preferably, several GPS pseudoranges are computed in parallel corresponding to several GPS satellites, respectively.
The digital signal processor preferably includes a numerically controlled signal generator, a frequency converter, a combiner, and a correlator for processing the stored raw GPS signal samples with several passes. In a first pass, the signal generator derives a local frequency from the reference frequency and adjusts the local frequency according to the frequency adjustment based on the radio signal frequency and GPS information from the base. The frequency converter uses the pre-adjusted local frequency for frequency converting the raw GPS signal samples to GPS signal code data at baseband. The combiner uses time synchronization based upon the time and location information from the base for combining epochs of GPS signal code data at like phase offsets within the epochs in order to form representative code epochs. Blocks of GPS signal code data that are combined are termed chunks. Preferably the chunks are about ten to twenty code epochs in length. The correlator correlates the chunk representative epochs to a replica code epoch and provides correlation levels and times. The microcontroller includes a calculator for interpolating the correlation times for the highest correlation levels, calculating a correlation time difference across the signal sample time period, and using the correlation time difference for determining a corresponding frequency correction.
In a second pass, the signal generator corrects the local frequency according to the frequency correction determined in the first pass. The frequency converter uses the corrected local frequency for re-converting the stored raw GPS signal samples to baseband for a second set of GPS signal code data. The combiner uses synchronization based upon the correlation times determined in the first pass and information for polarities of GPS data received from the base for combining epochs of the second GPS signal code data at like phase offsets within the epochs in order to form representative code epochs for larger blocks of GPS signal code data termed super chunks. Preferably the super chunks are about one-hundred to one-thousand epochs in length. The correlator correlates the super chunk representative epochs to the replica epoch and provides correlation levels and times. The calculator interpolates the correlation times for the highest correlation levels, calculates the difference in correlation times across the signal sample time period, and uses the correlation time difference for determining a second frequency correction. The first and/or the second passes may be repeated until frequency corrections are less than pre-determined thresholds.
In a third pass, the signal generator corrects the local frequency according to the frequency correction determined in the second pass. The frequency converter uses the second pass corrected local frequency for re-converting the stored raw GPS signal samples and providing a third set of GPS signal code data. The combiner combines epochs of the third GPS signal code data at like phase offsets within the epochs for all the epochs in the third GPS signal code data into one combined representative epoch. The correlator correlates the combined representative epoch to the replica epoch for determining combined correlation times and levels. The calculator interpolates the correlation times for the highest correlation levels and uses the interpolated correlation time for determining the GPS pseudorange. The GPS pseudoranges to several GPS signal transmitters may be determined in this way in parallel or the frequency corrections and correlation times for a first acquired GPS transmitter may be used to speed the acquisition of subsequent GPS transmitters. Rapid GPS signal acquisition is obtained because only one time period of raw GPS signal samples is required to be received from the GPS signal. Then, raw GPS signal samples may be processed by the digital signal processor and microcontroller at a rate that is fast compared to the rate at which new time periods would otherwise be received in the GPS signal.
In general, the signal-to-noise ratio at which the incoming GPS signal can be acquired depends upon the length of the chunks because the process of combining the epochs tends to reinforce the signal energy while averaging the noise energy to zero. The longer the chunks, the lower the incoming signal-to-noise ratio at which the GPS signal can be acquired. For example, processing a representative epoch for a chunk of ten epochs enables the GPS receiver of the present invention to acquire an incoming GPS signal at about ten decibels lower signal-to-noise as compared to a GPS receiver processing single epochs while processing a representative epoch for a chunk of one-hundred epochs enables the GPS receiver of the present invention to acquire an incoming GPS signal at about twenty decibels lower signal-to-noise. However, there is a limit to the length of the chunks that may be processed that is determined by the difference between the frequency of the incoming GPS signal and the local frequency within the GPS receiver that is used for converting the GPS signal to baseband. This can be understood by noting that the frequency difference between the local frequency and the GPS frequency causes inversions in the GPS signal code data at a time period of one-half the inverse of the frequency difference. For example, a frequency difference of fifty Hertz causes the baseband GPS signal code data to invert each ten milliseconds. When one of these inversions occurs within a chunk, a given length of the GPS signal code data before the inversion cancels the signal energy in the same length of the GPS signal code data after the inversion, thereby reducing or eliminating the utility of combining epochs of GPS signal code data to obtain a representative epoch. For a fifty Hertz accuracy for the pre-adjusted local frequency (ten millisecond inversions) a chunk can include ten epochs (ten milliseconds) because either I GPS signal code data or Q GPS signal code data will typically have a preponderance of GPS signal code data of the same inversion. For a five Hertz accuracy, a chunk can include one-hundred epochs (one-hundred milliseconds); or for a one-half Hertz accuracy a chunk can include one-thousand epochs (one second). Accordingly, in a preferred embodiment, when the radio signal frequency and the GPS information enable the local frequency to be accurate to about fifty Hertz, the GPS receiver processes chunks of ten epochs for determining correlation times for correcting the local frequency. When the local frequency is accurate to about five Hertz, the GPS receiver processes super chunks of about one-hundred epochs for determining more accurate and less noisy correlation times for further correcting the local frequency. When the local frequency is accurate to about one-half Hertz the GPS receiver processes a combined one-thousand epochs to determine a still more accurate and less noisy correlation time. The pseudorange to the transmitter of the incoming GPS signal is mathematically derived from the correlation time. In a preferred embodiment the correlation time for one-thousand epochs or one second of GPS signal code data is used for determining a pseudorange having sufficient accuracy and low noise.
In one preferred embodiment, the GPS receiver includes a GPS antenna having a substantially omni-directional radiation pattern. Such GPS antenna necessarily has a low antenna gain. However, the GPS receiver of the present invention has an improved capability for acquiring a GPS signal having a low signal level, thereby enabling the present GPS receiver to use a GPS antenna having a low antenna gain for receiving an airwave GPS signal.
An advantage of the GPS receiver of the present invention is that rapid GPS signal acquisition is obtained for a GPS signal having a low signal-to-noise ratio by combining chunks of epochs of GPS signal code data, thereby improving the likelihood that the GPS signal can be acquired indoors or beneath foliage.
Another advantage of the GPS receiver of the present invention is that a substantially omni-directional antenna can be used, thereby enabling a user to use the GPS receiver without regard to the directional orientation of the GPS receiver.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various figures.